An important component of disposable absorbent articles such as diapers is an absorbent core structure comprising superabsorbent polymers, typically hydrogel-forming superabsorbent polymers, also referred to as absorbent gelling material (AGM), or super-absorbent polymers. This polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness.
Superabsorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or poly-functional monomers such as N,N′-methylene-bis-acrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to lightly cross-link the polymer chains thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked superabsorbent polymers contain a multiplicity of neutralized carboxylate groups attached to the polymer backbone. It is generally believed, that these carboxylate groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network.
Typically, the superabsorbent polymers are provided in particulate form.
In addition, the superabsorbent polymer particles are often treated as to form a surface cross-linked layer on the outer surface in order to improve their properties in particular for application in baby diapers.
Generally, superabsorbent polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen superabsorbent polymer particles to resist deformation under an applied stress in the absorbent article. The gel strength needs to be high enough in the absorbent article so that the superabsorbent polymer particles do not deform and fill the capillary void spaces to an unacceptable degree causing so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake or the fluid distribution, i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article and leakage from the absorbent article can take place well before the superabsorbent polymer particles are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article. Thus, it is important that the superabsorbent polymer particles (when incorporated in an absorbent structure or article) maintain a high wet-porosity and have a high resistance against deformation thus yielding high permeability for fluid transport through the swollen gel bed.
Superabsorbent polymers with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure such as the pressure caused by the wearer, but this typically also reduces the absorbent capacity of the gel undesirably.
Often the surface crosslinked superabsorbent polymer particles are constrained by the surface-crosslinking ‘shell’ and cannot absorb and swell sufficiently, and/or that the shell is not strong enough to withstand the stresses of swelling or the stresses associated with performance under load.
It has been proposed in recent years to coat superabsorbent polymer particles with elastomeric film forming polymers, for example in WO2005/014065.
The inventors have developed new superabsorbent polymer particles coated with a specific block copolymer as described herein. The resulting coated superabsorbent polymer particles have been found to show a good transportation of water based liquids through the coating into the superabsorbent polymer particles, a good stability of the coating upon expansion of the superabsorbent polymers and a good liquid transport between the coated superabsorbent particles. These properties in combination improve liquid transport and absorption properties of the coated superabsorbent particles described herein.